As LSI circuits are increasing in density, the line width of circuits of semiconductor devices is becoming finer. To form a desired circuit pattern onto a semiconductor device, a method of reducing and transferring, by using a reduction-projection exposure apparatus, onto a wafer a highly precise original image pattern (mask, or reticle, in particular, when used in a stepper or a scanner) formed on a quartz is employed. The highly precise original image pattern is written by using an electron beam writing apparatus, in which a technology commonly known as electron beam lithography is used.
In the electron beam writing apparatus, a phenomenon called beam drift may occur in which, due to various factors, the irradiation position of the electron beam shifts over time during writing. A drift correction is made to cancel out the beam drift. In the drift correction, a measurement mark on a mark substrate disposed on a stage is scanned with an electron beam, and the irradiation position of the electron beam is measured to determine the amount of drift. The measurement mark is, for example, a metal pattern of dots or crosses.
Generally, each sample (mask), which is a writing target, has a different thickness because of the tolerance. The mark substrate placed on the stage is warped or tilted. The resulting difference between the surface height of the mark substrate and the surface height of the sample causes an error between the amount of drift on the mark surface and the amount of drift on the sample surface. To make a drift correction with high accuracy, it is necessary to consider the difference between the height of the mark surface and the height of the sample surface.
The electron beam writing apparatus includes a detector that irradiates the sample surface and the mark surface with laser light so as to detect the height of the sample surface and the height of the mark surface from the corresponding light receiving position of the reflected light. In the mark substrate, if a pattern region having the measurement mark formed therein is irradiated with laser light, light interference or the like may cause an error in height measurement. Accordingly, the detector irradiates a non-pattern region having no pattern therein with laser light.
However, warpage or tilt of the mark substrate creates a height difference between the pattern region and the non-pattern region. This results in an error between the surface height of the mark substrate detected by the detector and the height of the mark surface where the amount of drift is measured. Therefore, it has been difficult to make a drift correction with high accuracy.